The invention relates to an illumination system for illuminating a display device, which illumination system is provided with a light-emitting panel comprising
a front wall, a rear wall situated opposite said front wall, and edge areas between the front wall and the rear wall,
at least one of the edge areas of the panel being light-transmitting,
at least one light source being associated with the light-transmitting edge area, and
in operation, light originating from the light source being incident on the light-transmitting edge area and spreading in the panel.
The invention also relates to a display device comprising said illumination system.
Such illumination systems are known per se and are alternatively referred to as edge lighting systems. They are used, inter alia, as a backlighting of (image) display devices, for example for television receivers and monitors. Such illumination systems can particularly suitably be used as backlights for non-emissive displays, such as liquid crystal display devices, also referred to as LCD panels, which are used in (portable) computers or (cordless) telephones.
Said display devices generally include a substrate provided with a regular pattern of pixels, which are each driven by at least one electrode. In order to reproduce an image or a datagraphic representation in a relevant area of a (display) screen of the (image) display device, the display device uses a control circuit. In an LCD device, the light originating from the backlight is modulated by means of a switch or a modulator, while applying various types of liquid crystal effects. In addition, the display may be based on electrophoretic or electromechanical effects.
In the illumination systems mentioned in the opening paragraph, customarily a tubular low-pressure mercury vapor discharge lamp, for example one or more cold cathode fluorescent lamps (CCFL) are used as the light source, the light which is emitted, in operation, by the light source being coupled into the light-emitting panel which serves as an optical waveguide. This optical waveguide generally forms a comparatively thin and flat panel which is made, for example, of a synthetic resin material or glass, light being transported through the optical waveguide under the influence of (total) internal reflection.
Such an illumination system may also be provided with an alternative light source in the form of a plurality of optoelectronic elements, also referred to as electro-optic elements, such as electroluminescent elements, for example light-emitting diodes (LEDs). These light sources are generally provided in the proximity of, or are contiguous with, a light-transmitting edge area of the light-emitting panel, so that, in operation, light originating from the light source is incident on the light-transmitting edge area and spreads in the panel.
WO-A 99 53 236 discloses an illumination system which causes an LCD panel to be illuminated at different types of ambient light. For the light source use is made of incandescent lamps which couple light into a light-emitting panel, also referred to as light pipe. In the light-emitting panel, multiple reflections of light bring about a light distribution to illuminate the LCD panel.
An illumination system of the above-mentioned type has the disadvantage that the light distribution in the light-emitting panel, particularly at the edges of the light-emitting panel, is insufficiently uniform. As a result, the illumination uniformity of the display device is insufficient.
It is an object of the invention to completely or partly overcome the above-mentioned drawback.
In accordance with the invention, this object is achieved in that
the light source associated with the light-transmitting edge area of the light-emitting panel comprises clusters of at least three light-emitting diodes having a first, a second and a third light emission wavelength, which light emission wavelengths are different,
an imaginary mirror surface, which relates to the cluster, being situated in the center of each cluster,
the light-emitting diodes in the cluster being arranged in a direction perpendicular to the imaginary mirror surface, and
the light-emitting diodes being distributed over the cluster in such a manner that the spectral emissions of the light emitted by the light-emitting diodes are mirror symmetrical with respect to the imaginary mirror surface.
The LEDs of different colors cause undesirable color effects near the edges of the light-emitting panel. In such illumination systems, generally, clusters of LEDs are applied in the ratio R (red):G (green):B (blue)=1:1:1 or R:G:B=1:2:1, dependent upon the relative luminous flux emitted, in operation, by the LEDs. The use of a larger number of LEDs per cluster is possible, however, this has the disadvantage that homogeneously mixing the colors becomes gradually more difficult. In the case of a linear array of said LEDs, the pitch between the clusters of LEDs has a strong influence on the amount of space that is necessary before the light is sufficiently mixed to achieve the desired color uniformity. Given the above-mentioned known R:G:B ratios, the length necessary to sufficiently mix the light amounts to three to four times the pitch between the clusters of LEDs.
However, at said known ratios R:G:B=1:1:1 or 1:2:1, angular effects occur at the edges of the light-emitting panel. Particularly, undesirable color effects occur near the edges of the light-emitting panel, and the color point depends on the position on the light-emitting panel. The radiation pattern of the LEDs influences said angular effects.
The inventors have recognized that these angular effects are reduced substantially if the clusters of LEDs used are symmetrical as regards their light emission wavelength. The edge areas of the light-emitting panel, which extend transversely to the light-transmitting edge area, act as a (specular and/or diffuse) mirror for the light emitted by the LEDs. If the LEDs are arranged in the clusters in such a manner that they are symmetrical in terms of light emission wavelength, then the mirror effect of the edge areas of the light-emitting panel, which edge areas extend transversely to the light-transmitting edge area, is reduced substantially.
Arranging the LEDs in a cluster in such a manner that they are symmetrical in terms of light emission wavelength, implies that LEDs of the same light emission wavelength are situated on both sides of the imaginary mirror surface. If there is an odd number of LEDs per cluster, then the imaginary mirror surface intersects the middle LED in the cluster.
By virtue of the measure in accordance with the invention, the uniformity of the distribution of the light emitted by the illumination system is improved. As a result, a more uniform illumination of the (image) display device is obtained.
Preferably, the light source comprises symmetric clusters of blue, green and red light-emitting diodes. Blue, green and red are the primary colors used in (image) display devices. Preferably, LEDs are used having a high light output and a comparatively broad radiation pattern. Furthermore, in order to improve the mixing of light, preferably LEDs emitting light in pronounced lobes are used.
Preferably, each one of the light-emitting diodes has a luminous flux of at least 5 lm. LEDs having such a high output are alternatively referred to as LED power packages.
The smaller the number of LEDs per cluster, the more compact the illumination system can be designed.
A preferred embodiment of the illumination system is characterized in that
each one of the clusters comprises five light-emitting diodes,
one light-emitting diode having the first light emission wavelength being situated in the center of the cluster,
said light-emitting diode being arranged between two light-emitting diodes having the second light emission wavelength, and
each of said two light-emitting diodes being flanked, on the side facing away from the center of the cluster, by a light-emitting diode having the third light emission wavelength.
It is particularly preferred to employ an illumination system comprising five LEDs per cluster, wherein the ratio R:G:B=1:2:2, R:G:B=2:1:2 or R:G:B=2:2:1. By way of example, the clusters mentioned herein comprise, successively, GBRBG, RGBGR or RBGBR LEDs. Clusters having such R:G:B ratios are favorable because the number of LEDs per cluster is comparatively small and variations in the output of the LED can be readily compensated for. This has the additional advantage that a slightly larger distance between the LEDs can be chosen.
An alternative embodiment of the illumination system is characterized in that
each one of the clusters comprises six light-emitting diodes,
two light-emitting diodes having the first light emission wavelength being situated in the center of the cluster,
said two light-emitting diodes being arranged between two light-emitting diodes having the second light emission wavelength, and
the latter two light-emitting diodes each being flanked, on the side facing away from the center of the cluster, by a light-emitting diode having the third light emission wavelength.
It is particularly preferred to employ an illumination system comprising six LEDs per cluster, wherein the ratio R:G:B is 2:2:2, for example clusters comprising successively GBRRBG, RGBBGR or BRGGRB LEDs.
A further alternative embodiment of the illumination system is characterized in that
each one of the clusters comprises seven light-emitting diodes,
one light-emitting diode having the first light emission wavelength being situated in the center of the cluster,
said light-emitting diode being arranged between two light-emitting diodes having the second light emission wavelength,
said two light-emitting diodes each being flanked, on the side facing away from the center of the cluster, by a light-emitting diode having the third light emission wavelength, and
the latter light-emitting diodes each being flanked, on the side facing away from the center of the cluster, by a light-emitting diode having the first light emission wavelength.
It is particularly preferred to employ an illumination system comprising seven LEDs per cluster, wherein the ratio R:G:B=2:2:3, R:G:B=2:3:2 or R:G:B=3:2:2. By way of example, the clusters mentioned here successively comprise GBRGRBG, RGBRBGR or BGRBRGB LEDs.
If a still larger number of LEDs per cluster is chosen, it is possible that a periodic pattern of colors forms at the edge of the illumination system as a result of the fact that the colors do not mix very well. Such periodicity generally corresponds to the pitch between the clusters.
In general, each cluster comprises an integral number of LEDs. In general, if each cluster comprises an even number of LEDs, the imaginary mirror surface is situated between two LEDs. Conversely, if each cluster comprises an odd number of LEDs, the imaginary mirror surface is situated, in general, in the center of one LED, i.e. the imaginary mirror surface xe2x80x9cintersectsxe2x80x9d the middle LED of the cluster. Correspondingly, it can be imagined that an imaginary interface extends between two clusters of LEDs. In general, such an imaginary interface extends between two LEDs, one of said LEDs forming part of one cluster and the other LED forming part of the other, neighboring cluster. In an alternative embodiment of the illumination system, the imaginary interface between two clusters of LEDs is situated in the middle of one LED, i.e. the imaginary interface of two clusters intersects said LED. Consequently, one half of said LED forms part of one cluster of LEDs and the other half of said LED forms part of the other, neighboring cluster of LEDs. As one LED is shared by two clusters, the number of LEDs can be reduced while the symmetry of the clusters is preserved.
An important aspect in the case of clusters of LEDs which share a LED at their interface is that the LED situated at the edge of the light-emitting panel only half contributes to the generation of light in the light-emitting panel. This can be attributed to the fact that, in principle, only one half of this LED forms part of its cluster, but said LED is not shared with another cluster, as in the case of the other clusters. This 50% contribution to the light generation in the light-emitting panel is achieved in a simple manner by screening one half of the LED situated at the edge of the light-emitting panel, or by supplying such a current to this LED that the light output of said LED is only 50% of the normal light output.
Thus, a preferred embodiment of the illumination system is characterized in that each of the clusters comprises four light-emitting diodes, and two neighboring clusters of LEDs share one LED, one light-emitting diode having the first light emission wavelength being situated in the middle of the cluster, said light-emitting diode being arranged between two light-emitting diodes having the second light emission wavelength, and said light-emitting diodes each being flanked, on the side facing away from the middle of the cluster, by a light-emitting diode having the third light emission wavelength, one half of the light-emitting diode having the third light emission wavelength forming part of the neighboring cluster of LEDs.
It is particularly preferred to employ an illumination system comprising four LEDs per cluster, wherein the ratio R:G:B=1:1:2, R:G:B=1:2:1 or R:G:B=2:1:1. By way of example, the clusters mentioned here successively comprise GBRBG, GRBRG, RGBGR, RBGBR, BGRBG or BRGRB LEDs. In all these examples, the first-mentioned and the last-mentioned LED form part of the neighboring cluster, the LEDs situated nearest to the edge of the light-emitting panel only half contributing to the relevant cluster in the manner described hereinabove. Clusters with such R:G:B ratios are favorable because the number of LEDs per cluster is comparatively small and variations in the output of the LED can be readily compensated for. This has the additional advantage that a slightly larger distance between the LEDs can be chosen.
Correspondingly, an alternative embodiment of the illumination system is characterized in that each of the clusters comprises five light-emitting diodes, and two neighboring clusters of LEDs share one LED, two light-emitting diodes having the first light emission wavelength being situated in the middle of the cluster, said light-emitting diodes being arranged between two light-emitting diodes having the second light emission wavelength, and the latter light-emitting diodes each being flanked, on the side facing away from the middle of the cluster, by a light-emitting diode having the third light emission wavelength, one half of the light-emitting diode having the third light emission wavelength forming part of the neighboring cluster of LEDs.
It is particularly preferred to employ an illumination system comprising five LEDs per cluster, wherein the ratio R:G:B=1:2:2, R:G:B=2:2:1 or R:G:B=2:1:2. By way of example, the clusters mentioned here successively comprise RBGGBR, GRBBRG or BGRRGB LEDs. In all these examples, the first-mentioned and the last-mentioned LED form part of the neighboring cluster, the LEDs situated closest to the edge of the light-emitting panel only half contributing to the relevant cluster in the manner described hereinabove.
Correspondingly, a further alternative embodiment of the illumination system comprises six LEDs per cluster, the ratios of the LEDs being R:G:B=1:2:3, R:G:B=2:3:1 or R:G:B 3:2:1, or the ratio of the LEDs being R:G:B=1:1:4, R:G:B=1:4:1 or R:G:B=4:1:1. By way of example of the former ratio, the clusters mentioned here successively comprise RGBGBGR or RBGGGBR LEDs. By way of example of the latter ratio, the clusters mentioned here successively comprise RGGBGGR LEDs. In all these examples, the first and the last LED form part of the neighboring cluster, the LEDs situated closest to the edge of the light-emitting panel only half contributing to the relevant cluster in the manner described hereinabove.
In an interesting, alternative embodiment of the illumination system, use is made of LEDs having a combination of two light emission wavelengths. This is achieved, for example, by partly providing the blue LEDs with a green phosphor, or by partly providing green LEDs with a red phosphor, or by partly providing blue LEDs with a red phosphor. In this manner, the symmetric clusters of LEDs can be formed using fewer LEDs.
To achieve this, an embodiment of the illumination system is characterized in that
the light source associated with the light-transmitting edge area of the light-emitting panel comprises clusters of at least one light-emitting diode having a first light emission wavelength and at least one light-emitting diode having a combination of a second and a third light emission wavelength, which light emission wavelengths are different,
an imaginary mirror surface that relates to the cluster being situated in the middle of each cluster,
the light-emitting diodes in the cluster being arranged in a direction perpendicular to the imaginary mirror surface, and
the light-emitting diodes being distributed over the cluster in such a manner that the spectral emissions of the light emitted by the light-emitting diodes are mirror symmetrical with respect to the imaginary mirror surface.
In said embodiment of the illumination system, preferably, each one of the clusters comprises three light-emitting diodes,
one light-emitting diode having the first light emission wavelength being situated in the middle of the cluster,
said light-emitting diode being arranged between two light-emitting diodes having the combination of the second and the third light emission wavelength.
In an alternative embodiment, one light-emitting diode having the second and the third light emission wavelength is situated in the middle of the cluster and is flanked, on each side, by a light-emitting diode having the first light emission wavelength.
Using three LEDs per cluster, a particularly compact illumination system having symmetric clusters of LEDs is obtained.
In said embodiment of the illumination system, preferably, each one of the clusters comprises four light-emitting diodes,
two light-emitting diodes having the combination of the first and the second light emission wavelength being situated in the middle of the cluster,
said light-emitting diodes being arranged between two light-emitting diodes having the first light emission wavelength.
In an alternative embodiment, two light-emitting diodes having the third light emission wavelength are situated in the middle of the cluster and are flanked, on each side, by a light-emitting diode having the combination of the first and the second light emission wavelength.
Using four LEDs per cluster, a very compact illumination system having symmetric clusters of LEDs is obtained.
In said embodiment of the illumination system, preferably, each one of the clusters comprises five light-emitting diodes,
one light-emitting diode having the combination of the first and the second light emission wavelength being situated in the middle of the cluster,
said light-emitting diode being arranged between two light-emitting diodes having the third light emission wavelength, and
said two light-emitting diodes each being flanked, on the side facing away from the middle of the cluster, by a light-emitting diode having the combination of the first and the second light emission wavelength.
In an alternative embodiment, a light-emitting diode having the third light emission wavelength is arranged in the middle of the cluster between two light-emitting diodes having the combination of the first and the second light emission wavelength, said two light-emitting diodes each being flanked, on the side facing away from the middle of the cluster, by a light-emitting diode having the first light emission wavelength.
Using five LEDs per cluster, a compact illumination system having symmetric clusters of LEDs is obtained.
In the above-mentioned, alternative embodiment of the illumination system, wherein the imaginary interface between two clusters of LEDs is situated in the middle of one LED, it is possible to further reduce the number of LEDs per cluster.
Thus, a preferred embodiment of the illumination system is characterized in that each of the clusters comprises two light-emitting diodes, and two neighboring clusters of LEDs share one LED, one light-emitting diode having the first light emission wavelength being situated in the middle of the cluster, said light-emitting diode being arranged between two light-emitting diodes having the combination of the second and the third light emission wavelength, one half of the light-emitting diode having the combination of the second and the third light emission wavelength forming part of the neighboring cluster of LEDs. In the opposite case, one light-emitting diode having the combination of the second and the third light emission wavelength is situated in the middle of the cluster, said light-emitting diode being arranged between two light-emitting diodes having the first light emission wavelength. In the latter case, (one half of) the light-emitting diodes having the first light emission wavelength form part of the neighboring clusters.
Correspondingly, an alternative embodiment of the illumination system is characterized in that each one of the clusters comprises three light-emitting diodes, and two neighboring clusters of LEDs share one LED, two light-emitting diodes having the first light emission wavelength being situated in the middle of the cluster, said two light-emitting diodes being arranged between two light-emitting diodes having the combination of the second and the third light emission wavelength, one half of the light-emitting diode having the combination of the second and the third light emission wavelength forming part of the neighboring cluster of LEDs. In the opposite case, two light-emitting diodes having the combination of the second and the third light emission wavelength are situated in the middle of the cluster, said two light-emitting diodes being arranged between two light-emitting diodes having the first light emission wavelength. In the latter case, (one half of) the two light-emitting diodes having the first light emission wavelength form part of the neighboring clusters.
Correspondingly, a further alternative embodiment of the illumination system is characterized in that each one of the clusters comprises four light-emitting diodes, and two neighboring clusters of LEDs share one LED, three light-emitting diodes having the first light emission wavelength being situated in the middle of the cluster, said three light-emitting diodes being arranged between two light-emitting diodes having the combination of the second and the third light emission wavelength, one half of the light-emitting diode having the combination of the second and the third light emission wavelength forming part of the neighboring cluster of LEDs. In the opposite case, three light-emitting diodes having the combination of the second and the third light emission wavelength are situated in the middle of the cluster, said three light-emitting diodes being arranged between two light-emitting diodes having the first light emission wavelength. In the latter case, (one half of) the two light-emitting diodes having the first light emission wavelength form part of the neighboring clusters.
The object in accordance with the invention is alternatively achieved in that
the illumination system comprises a first light-emitting panel and a second light-emitting panel, said first and second light-emitting panels being arranged at least substantially parallel to each other,
the light source associated with the light-transmitting edge area of the first light-emitting panel comprises a plurality of light-emitting diodes having a first light emission wavelength, and
the light source associated with the light-transmitting edge area of the second light-emitting panel comprises clusters of at least two light-emitting diodes having a second and a third light emission wavelength,
an imaginary mirror surface that relates to the cluster being situated in the middle of each cluster,
the light-emitting diodes in the cluster being arranged in a direction perpendicular to the imaginary mirror surface,
the light-emitting diodes being distributed over the cluster in such a manner that the spectral emissions of the light emitted by the light-emitting diodes are mirror symmetrical with respect to the imaginary mirror surface,
and the first, the second and the third light emission wavelength being different.
The advantage of applying more than one light-emitting panel in the illumination system is that the light-transmitting edge areas of the first light-emitting panel are associated with (at the most) two LEDs having different light emission wavelengths, and that the light-transmitting edge areas of the second light-emitting panel are associated with one or (at the most) two LEDs. If three LEDs of different colors are associated with the light-transmitting edge area of a light-emitting panel, comparatively much more space is necessary to sufficiently mix the light originating from the LEDs. By coupling light originating from maximally two types of LEDs into one light-emitting panel, and by coupling light originating from maximally two types of LEDs into the other light-emitting panel, the space necessary to mix light is reduced substantially.
A further advantage of the use of multiple light panels is that the light output and the light distribution of each of the panels can be individually influenced.
The light source associated with the light-transmitting edge area of the first light-emitting panel preferably comprises a plurality of green light-emitting diodes, and the light source associated with the light-transmitting edge area of the second light-emitting panel preferably comprises symmetric clusters of blue and red light-emitting diodes.
A preferred embodiment of the illumination system is characterized in that
each one of the clusters comprises three light-emitting diodes,
one light-emitting diode having the second light emission wavelength being situated in the middle of the cluster,
said light-emitting diode being arranged between two light-emitting diodes having the third light emission wavelength.
It is particularly preferred to employ an illumination system comprising three LEDs per cluster, wherein the ratio between the numbers of LEDs associated with the light-transmitting edge area of the second light-emitting panel is 1:2. If, by way of example, only G LEDs are associated with the light-transmitting edge area of the first light-emitting panel, then clusters of successively BRB or RBR are associated with the ligh-transmitting edge area of the second light-emitting panel.
In the above-mentioned alternative embodiment of the illumination system, wherein the imaginary interface between two clusters of LEDs is situated in the middle of one LED, the number of LEDs per cluster can be further reduced.
Thus, a preferred embodiment of the illumination system is characterized in that each one of the clusters comprises two light-emitting diodes, and two neighboring clusters of LEDs share one LED, one light-emitting diode having the second light emission wavelength being situated in the middle of the cluster, said light-emitting diode being arranged between two light-emitting diodes having the third light emission wavelength, one half of the light-emitting diode having the third light emission wavelength forming part of the neighboring cluster of LEDs. In the opposite case, one light-emitting diode having the third light emission wavelength is situated in the middle of the cluster, said light-emitting diode being arranged between two light-emitting diodes having the second light emission wavelength. In the latter case, (one half of) the light-emitting diodes having the second light emission wavelength form part of the neighboring clusters.
In an alternative embodiment, both the first and the second light-emitting panel are provided with clusters of three LEDs. By way of example, clusters of successively GBG LEDs are associated with the first light-emitting panel, and clusters of successively RGR LEDs are associated with the second light-emitting panel. In this manner, a ratio of 1:2:3 between the total number of LEDs can be achieved. In a further example, clusters of successively GBG LEDs are associated with the first light-emitting panel, and clusters of successively RBR LEDs are associated with the second light-emitting panel. In this manner, a ratio of 1:1:1 between the total number of LEDs can be achieved.
An alternative embodiment of the illumination system is characterized in that
each one of the clusters comprises four light-emitting diodes,
two light-emitting diodes having the second light emission wavelength being situated in the middle of the cluster,
said light-emitting diodes being arranged between two light-emitting diodes having the third light emission wavelength.
It is particularly preferred to employ an illumination system comprising four LEDs per cluster, wherein the ratio between the numbers of LEDs associated with the light-transmitting edge area of the second light-emitting panel is 2:2. If, by way of example, only G LEDs are associated with the light-transmitting edge area of the first light-emitting panel, then clusters of successively BRRB or RBBR are associated with the light-transmitting edge area of the second light-emitting panel.
A further alternative embodiment of the illumination system is characterized in that
each one of the clusters comprises five light-emitting diodes,
one light-emitting diode having the third light emission wavelength being situated in the middle of the cluster,
said light-emitting diode being arranged between two light-emitting diodes having the second light emission wavelength, and
said two light-emitting diodes each being flanked, on the side facing away from the middle of the cluster, by a light-emitting diode having the third light emission wavelength.
It is particularly preferred to employ an illumination system comprising five LEDs per cluster, wherein the ratio between the numbers of LEDs associated with the light-transmitting edge area of the second light-emitting panel is 2:3. If, by way of example, only G LEDs are associated with the light-transmitting edge area of the first light-emitting panel, then clusters of successively BRBRB or RBRBR are associated with the light-transmitting edge area of the second light-emitting panel.
A further alternative embodiment of the illumination system is characterized in that
each one of the clusters comprises five light-emitting diodes,
one light-emitting diode having the second light emission wavelength being situated in the middle of the cluster,
said light-emitting diode being arranged between two light-emitting diodes having the third light emission wavelength, and
said two light-emitting diodes each being flanked, on the side facing away from the middle of the cluster, by a light-emitting diode having the third light emission wavelength.
It is particularly preferred to employ an illumination system comprising five LEDs per cluster, wherein the ratio between the numbers of LEDs associated with the light-transmitting edge area of the second light-emitting panel is 1:4. If, by way of example, only G LEDs are associated with the light-transmitting edge area of the first light-emitting panel, then clusters of successively BBRBB or RRBRR are associated with the light-transmitting edge area of the second light-emitting panel.
In alternative embodiments of the illumination system, the colors associated with the light-emitting panels are interchanged.
The amount of light emitted by the LEDs is regulated by varying the luminous flux of the light-emitting diodes. Regulating the luminous flux generally takes place in an energy-efficient manner. For example, LEDs can be dimmed without a noticeable decrease in efficiency. Preferably, the intensity of the light emitted by the light-emitting diodes is variable in response to the illumination level of an image to be displayed by the display device, or in response to the level of the ambient light. Preferably, the color point of an image displayed by the display device is determined by the illumination system. By virtue thereof, a(n) (improved) dynamic range (for example contrast) of the image to be displayed by the display device is obtained.
By virtue of the measure in accordance with the invention, the light emitted by the illumination system is more uniformly distributed. As a result, a more uniform illumination of the (image) display device is obtained. In addition, an illumination system in accordance with the invention enables a (laterally) more compact illumination system having the same light uniformity to be obtained.
In further alternative embodiments of the illumination system, apart from LEDs having a specific light emission wavelength, LEDs are used which are provided with a phosphor, as a result of which the light emission of the light emitted by the LED is converted by said phosphor to light having a different, desired light emission wavelength. A combination of red LEDs and phosphor LEDs can particularly suitably be used to produce the other colors.
Preferably, each one of the light-emitting diodes has a luminous flux of at least 5 lm. LEDs having such a high output are also referred to as LED power packages. The use of these high-efficiency, high-output LEDs has the specific advantage that, at a desired, comparatively high light output, the number of LEDs may be comparatively small. This has a positive effect on the compactness and the efficiency of the illumination system to be manufactured.
The use of LEDs has the further advantage that dynamic illumination possibilities are obtained. For this purpose, a sensor for measuring the optical properties of the light emitted, in operation, by the light source is preferably situated at a light-transmitting edge area. If different types of LEDs are combined and/or LEDs of different colors are employed, it is possible to mix colors in a desirable manner, for example, to enable the illumination system to emit white light of the desired color temperature. In addition, color changes can be brought about irrespective of the condition of the display device.
In order to save space, the light panels are preferably arranged one behind the other. The front wall or, preferably, the rear wall of the light-emitting panels is provided with means for coupling light out of the panel. These means for coupling out light are alternatively referred to as coupling-out members. These means, which are known per se, comprise (patterns of) deformations. Said means couple light out of the light-emitting panels by reflection, scattering and/or refraction. Generally, the means for coupling out light are distributed non-uniformly over the rear wall of the relevant light-emitting panel, i.e. they are provided at a predetermined gradient allowing light to be coupled out of the relevant light-emitting panel as uniformly as possible.
The first light-emitting panel, which is situated closest to the display device, allows passage of light originating from the second light-emitting panel, which is situated on the side of the first light-emitting panel facing away from the display device. In an alternative embodiment, the second light-emitting panel is arranged between the first light-emitting panel and the display device.
It is particularly favorable if the light-transmitting edge areas are alternately situated at opposite sides of the first and the second light-transmitting panel. This enables any influence of the light originating from gradients in the distribution of the means for coupling light out of the light-emitting panel to be compensated since the gradients extend in the same (yet opposite) direction. This can be achieved in a similar manner for three sequentially arranged light-emitting panels.
Preferably, the illumination system comprises control electronics for changing the luminous flux of the light-emitting diodes. Suitable control electronics enables the desired color temperature effects to be achieved. It is particularly favorable if the control electronics can be influenced by the user of the assembly, by means of a sensor that measures, for example, the color temperature of the ambient light, or by means of a video card of, for example, a (personal) computer and/or by means of drive software of a computer program.
In a further preferred embodiment of an illumination system according to the invention the clusters of LEDs are formed as multi-chip package containing the required number of light emitting diodes of the first, second and third emission wavelength and showing mirror symmetry of a imaginary mirror surface. Accordingly a preferably display device is provided with the multi-chip package LED clusters. This has the advantage that mixing of colors already starts inside the chip package, which has an improving effect on the mixing inside the light-emitting panel. Thus both color mixing as well as homogeneity of the light over the emitting panel surface is improved.